Accurate pressure measurements in high temperature applications, such as in the gas path of an aircraft engine, are required in order to monitor and improve the fuel efficiency, performance, and reliability of the engine. Gas path pressure measurements in severe environments have traditionally been performed through the measurement of the deflection of metallic diaphragms. The resulting mechanical deflection of the diaphragm is converted into an electrical signal by several approaches. One method utilizes a resistive strain gage mounted to the center of the diaphragm. Another method utilizes the change in capacitance between the moving diaphragm and a fixed reference surface. Both of these approaches produce acceptable results at relatively low temperatures, however, at temperatures in excess of 500.degree. C., the creep of the metallic diaphragm accelerates which results in a long-term drift of the pressure transducer output signal versus pressure calibration curve. In addition, it has been found that hysteresis in this calibration curve may become significant when these pressure transducers are operated at these high temperatures.
In order to reduce or eliminate the undesirable creep and hysteresis effects exhibited by metallic diaphragms at high temperatures, alternate diaphragm materials with improved high-temperature properties must be utilized. For example, various types of glasses and glass ceramics have excellent dimensional stability and these materials can replace metal as the material for pressure transducer diaphragms. Unfortunately, the hardness and rigidity of these materials, along with their inherent brittleness, dictate a diaphragm design that results in a smaller deflection with pressure than the deflection achievable with metallic diaphragms. These smaller deflections, in turn, necessitate the use of sensing techniques having significantly increased sensitivity so that the deflections can be measured. Such icreased sensitivity allows the measurements to be affected by dynamic vibration and temperature changes which may result in inaccurate measurements of diaphragm deflection.
Because of this, it has become desirable to develop a diaphragm-type pressure transducer and associated diaphragm deflection sensing apparatus that can be used in a high temperature environment, is sensitive to relatively small diaphragm deflections, and is unaffected by dynamic vibration and temperature changes.